1. Field of the Invention
The present invention relates generally to an engine valve train and more specifically to a variable valve timing arrangement which permits the lift and/or timing of the valves to be selectively varied.
2. Description of the Prior Art
JP-A-63-167016 and JP-A-63-57805 disclose rocker arm arrangements which include a main rocker arm which cooperates with a low speed cam and a sub-rocker arm which cooperates with a high speed cam. The two rocker arms are pivotally mounted on a common rocker arm shaft.
A hydraulically operated connection or interlocking device which enables the main and sub-rocker arms to be selectively locked together, comprises a set of plunger bores which are formed in the rocker arms in a manner to be parallel with and at a predetermined distance from, the axis of the shaft about which the arms are commonly pivotal. By applying a hydraulic pressure to the end or ends of the plungers reciprocally disposed in the bores, the plungers can be induced to move axially and induce the situation wherein two of the plungers will partially enter an adjacent bore and lock the two arms together.
However, these type of arrangement have suffered from the drawback that when the engine is being started the amount of hydraulic pressure produced by the engine oil pump is so low that sufficient bias cannot be produced to bias the plungers into their respective locking positions, the rocker arms cannot be locked together in a manner which prevents the relative movement therebetween.
This leads to the situation wherein, if the main rocker arm is arranged to be initially motivated by a so called "partial" cam (viz., a cam which features a profile designed to reduce the fuel consumption), the adiabatic expansion of the fuel during the induction phase is such that the ignition of the air-fuel charge is difficult and/or leads to incomplete combustion and formation of smouldering deposits on the spark plug terminals during start-up of a cold engine.
A further problem come in that, if a so called partial cam is used and the hydraulic control system malfunctions in a manner wherein an adequate level of hydraulic pressure cannot be supplied to the interlocking device, the amount of torque produced by the engine falls to a level wherein vehicle drivability is badly deteriorated.
Accordingly, it is extremely difficult in view of the above conflicting requirements, to set the profile of the cam which is initially used to motivate the rocker arm in a manner which suitably achieves the desired fuel economical operation during very low engine speed operation while enabling the engine to be readily started and/or produce adequate amounts of torque in the event of hydraulic failure.